I'm very surprised the unions are tolerating this to be honest. They've been very protective in the past of a fear Cityflyer pilots may be used on 'mainline' operations to undercut mainline T&Cs.
If this happens and is successful it's only a matter of time, IMO, before they look to do similar at Heathrow short haul before moving onto the long haul stuff.
Standard divide and conquer from IAG.